1. Field of the Invention
This invention relates to solutions of nickel hypophosphite having an increased nickel concentration. In one embodiment, the invention relates to make-up and replenishing solutions for electroless nickel baths that utilize nickel hypophosphite, the solutions having a nickel ion concentration above 36 grams per liter.
2. Description of the Prior Art
Nickel hypophosphite solutions are known, as is their use in electroless nickel baths for depositing nickel or a nickel alloy onto a substrate.
Electroless nickel plating is a widely utilized plating process which provides a continuous deposit of a nickel metal or alloy coating on metallic or non metallic substrates without the need for an external electric plating current.
Electroless nickel plating is described generally as a controlled autocatalytic chemical reduction process for depositing the desired nickel metal and is simply achieved by immersion of the desired substrate into an aqueous plating solution under appropriate electroless plating conditions.
In conducting electroless nickel plating, particularly from a bath which utilizes a hypophosphite as the reducing agent, the bath basically contains a source of nickel cations, usually nickel sulfate, and a hypophosphite reducing agent, usually sodium hypophosphite. The deposition reaction takes place in the bath and generally involves the reduction of a nickel cation to form a nickel metal alloy as a deposit on the desired substrate surface. The reduction reaction is generally represented by the following equation: EQU 3H.sub.2 PO.sub.2.sup.- +Ni+2.fwdarw.3/2H.sub.2 .uparw.+H.sup.+ +2HPO.sub.3.sup.-2 +P+Ni.sup.o
As the reaction continues the by-products and bath conditions created thereby present problems which adversely affect the desired plating process.
These problems include the buildup of the anion of the nickel salt employed to maintain the optimum nickel concentration, typically sulfate, as well as the increased concentration of extraneous sodium cations, from the use, of example, of sodium hydroxide to adjust the pH and sodium hypophosphite to replace the hypophosphite. This build-up or increase in the concentration of such anions and cations as they accumulate in the bath produces a deleterious effect on the plating reaction and adversely affects the quality of the plating deposited on the substrates. The accumulation of ionic species in the bath degrades the quality of the nickel deposit and makes it unacceptable for such high-level applications as hard discs for computers, as well as CD-ROM and other optical disc storage. Further, the anions adversely affect the bath by causing the bath to become unacceptable at low levels of metal turnover, i.e., the number of times that the original nickel source is replenished. Thus the accumulation of added sodium cations and sulfates prevents the long-term and economical use of the expensive plating solutions and adversely affects the nickel deposit.
To address these problems, it has been proposed to use nickel hypophosphite as the source of both nickel and hypophosphite in the bath.
For example, Nobel, et al., in U.S. Pat. No. 5,522,972 disclose that nickel hypophosphite is useful in the formulation of electroless nickel plating baths, particularly since one can eliminate both sodium and sulfate ions to give the bath longer life. Nobel, et al., disclose a method for preparing nickel hypophosphite by contacting a nickel anode with a solution containing hypophosphite ions and applying a current to dissolve the nickel into the solution and form a nickel hypophosphite solution.
This approach has the advantage that no extraneous ions are introduced into the bath. Since there is no sulfate ion in the bath there is no formation of sludge when orthophosphite is removed by precipitation with calcium. Also, since no sodium is introduced into the bath, the bath remains free of undesirable ionic species. However, the poor solubility of nickel metal in hypophosphorous acid--35 grams per liter at saturation--leads to problems because a considerable amount of nickel hypophosphite solution must be added to the plating tank to maintain the desired amount of nickel ion in the bath. At present, in standard electroless nickel systems using nickel sulfate as the nickel source, a make-up and replenishment solution is used that contains from about 75 to 100 grams per liter of nickel. Because of the low solubility of nickel in hypophorous acid--35 grams per liter at saturation, one must add a considerably greater volume of a nickel hypophosphite solution than nickel sulfate solution to the plating tank to maintain the desired amount of nickel ion in the bath. The large volume of nickel hypophosphite solution needed causes the tank to quickly fill to capacity and sometimes causes the electroless nickel solution to actually spill over the top of the vessel.
Another problems resulting from the large replenishment volumes required is that the operating temperature of the bath is rapidly changed. This, in turn, causes a change in the phosphorous content of the electroless nickel deposit and, therefore, significant variability in the quality of the deposit; the phosphorous content of the deposit increases with reduced temperature. In operation, a large replenishment could drop the operating temperature from 90.degree. C. to 85.degree. C. or less and it could take 10 minutes or more for the temperature to climb back to 90.degree. C., at which time another replenishment might be required.
It would be useful in connection with the various processes that utilize nickel hypophosphite, particularly with electroless nickel solutions, to have a solution with a nickel concentration above 35 grams per liter.